In some environments, such as the cockpits of aircraft, it is desirable to illuminate devices, such as instruments and displays, without increasing the level of ambient light in the cockpit. Increasing the level of ambient light sufficiently to view such devices will destroy the night vision of the pilot of the aircraft in this environment creating a safety hazard. As a result aircraft instruments are often illuminated by red lamps placed around the perimeter of an instrument but screened from direct view by the instrument panel, a border or a frame. This compromise creates adequate illumination at the edges of the instrument but often leads to inadequate or dim lighting of the central part of the instrument.
There are other environments, such as liquid crystal displays with maps, photographs, charts and the like placed behind the liquid crystal display which have problems with lighting because of the high loss of light as it passes and repasses through the liquid crystal display. In a liquid crystal display 75% of the ambient light is lost as it passes through the display due to the polarization which occurs therein; if this light is reflected back off of a depolarizing surface, such as paper, through the display in the reverse direction, less than 25% of the initial 25% of light, or 6% of the total light passing into the display will emerge from the display. For this reason liquid crystal displays are often back-lighted or back-lit to make them operable.
Problems with illumination in these environments have been addressed with edge-lit thin panel illuminators for instruments, maps, charts and the like where ambient light is unsuitable, unavailable or unusable. One of such devices is disclosed in U. S. Pat. No. 4,373,282 issued to Robert Wragg. In U.S. Pat. No. 5,089,916 issued to Lawrence Holmes, Jr. an edge-lit light wedge is employed to front light a chart or a map and to back light a liquid crystal display to obtain a functional navigational instrument.
Of course there are other devices employing edge-lit devices such as signs of the type shown in U.S. Pat. No. 4,385,343 issued to George W. Plumly and greeting cards having edge-lit displays as shown in U.S. Pat. No. 4,975,809 issued to Mihg-ho Ku.
The current invention relates to an improved edge-lit light wedge, referred to as a pane herein, which is suitable for the illumination of the navigational instrument described by Holmes in his patent and for other applications, such as liquid crystal displays with or without a map or chart underlaying such displays. For example liquid crystal displays or screens of portable computers can employ this novel edge-lit light pane to uniformly back light the screen leading to thinner display devices without any loss of performance.
An advantage of this novel invention is the ability to provide a highly efficient edge-lit pane which can be manufactured economically.
Another advantage of this novel invention is that its increased efficiency which allows it to be employed in applications which were previously unsuitable for such devices.
To achieve these advantages the novel invention employs a general configuration similar to the edge-lit light wedge described in the Holmes Patent, supra, but with modifications which greatly enhance its performance.